1. Field of the Invention
This invention relates generally to the use of optical devices to sense the progress of processes, transmit signals related to such progress, detect such signals and extract from such signals information to control such processes. More particularly, this invention relates to the use of an endpoint controller to control photoresist development in a multiple spray-puddle process for the fabrication of microelectronic devices, including, but not limited to semiconductor devices.
2. Brief Description of the Prior Art
Photolithography makes it possible to transfer a desired circuit pattern to a surface of a semiconductor device. In a simplified photolithograpic process a silicon or gallium arsenide wafer or other substrate with a suitable substrate coating such as silicon dioxide, polysilicon, or aluminum or other metal is coated with a photoresist film and then subjected to an imaging and developing process which exposes regions of the substrate coating in a pattern defined by a mask having opaque and transparent portions positioned to form the desired pattern. The wafer is then etched by a subsequent process in the pattern formed by the developing process.
To maximize the yield of useful devices, the photolithography process must be controlled and certain dimensions, known as critical dimensions, kept within specified ranges.
The point at which photolithographic reactions proceed far enough to expose the underlying material or film is commonly called endpoint. However, for reasons which will become apparent that point is more accurately called breakthrough.
The use of devices to detect the breakthrough point in the photoresist development phase of the manufacturing the process has been shown to significantly improve critical dimension control. This approach has been successfully used with constant spray processes and with spray-puddle processes(processes within which a developer is first sprayed upon the wafer and then allowed to puddle on the surface of the wafer) with a short spray period. A short spray period is defined as a spray period that consistently ends prior to breakthrough. It is desirable to use spray-puddle processes as they permit the conservative use of more expensive, developers, such as Tetramethylammonium Hydroxide, rather than Sodium or Potassium Hydroxide, which can contaminate the wafer. In addition spray-puddle process generally provide a more uniform development. As geometries get smaller, process latitude could be increased by diluting the developer, however the diluted developer tends to run out of active chemicals before the develop is complete. Thus multiple spray-puddle periods are used as a way of replenishing the developer. However attempts to use endpoint control devices within processes which include double or triple spray puddle stages, i.e. spray-puddle-spray-puddle or spray-puddle-spray-puddle-spray-puddle, have been generally unsuccessful because the change from spray to puddle or vice versa frequently occurs at about the same time within the process when breakthrough can be expected and tends to mask the signal. In addition, the change from spray to puddle or vice versa sometimes significantly changes the reflectivity of the wafer and such reflectance can be read as a false breakthrough point or prevent the detection of the real breakthrough point.
Various methods have been used to overcome the problems described above. For example, a constant spray process using a specialized low volume ultrasonic spray nozzle has been utilized. In one approach a multi-step detection process, changing steps whenever the process changes from spray to puddle or vice versa has been employed. Attempts have been made to overprocess based upon a percentage of step time or a percentage of the recipe time, however this approach only works if breakthrough occurs within finite time limits within a spray or puddle, and may require undesirable modification of the development process to meet the endpoint controller's requirement. In another approach the practice has been to increase the dynamic range of the endpoint controller to include both spray and puddle without changing steps. This method suffers when used with wafers where a small percentage of the wafer area has been exposed, as the signal cannot be both enlarged enough to see changes in the signal and also retain both the spray and the puddle signals on the monitor screen of the endpoint controller. Neither method solves the problem of missing breakthrough, if breakthrough occurs during a change from spray to puddle or vice versa, or the problem of reading a change as a false breakthrough point. Furthermore, none of these methods have been found to improve critical dimension uniformity, from wafer to wafer, enough to warrant the necessary recalibration of the process.
As a result of the above described problems, endpoint control of the develop process has been limited to those processes using immersion develop or track develop with either constant spray or a single spray-puddle step with a short spray. Since approximately 50% or more of existing track develop applications involve a multiple spray-puddle step, it is highly desirable to have an endpoint controlled development process which is useful with multiple spray-puddle steps. It is thus the primary object of this invention to provide a method for using process changes, including breakthrough detection, that is applicable to multiple spray-puddle photoresist develop applications and allows the operator or equipment to decide in real time when to go from puddle to spray and vice versa.
It is well known that wafers are not uniform from wafer to wafer. Because of this lack of uniformity, a system which uses fixed times to control the length of spray and puddle steps within a develop process cannot provide consistent development and thus perpetuates nonuniformity.
As discussed above, a problem with using endpoint detection to control spray-puddle processes, is the commonly occurring coincidence in time of breakthrough with a transition from spray to puddle. A simplistic solution to this problem would suggest that the process times be adjusted to avoid the anticipated time of breakthrough. However, the develop steps and the timing of such steps, are merely parts of a whole process, the design of which has been determined, both scientifically and empirically to provide an acceptable yield. If changes are made in the lengths of the times of the interim steps, the entire process would have to be recharacterized and the critical dimensions reanalyzed and recalibrated. For example, change in develop time will change the linewidth, an important critical dimension. An overall goal of fabricators in this field is to hold everything constant and thus proposals to change develop times are generally deemed to be undesirable. It is another object of this invention to provide a method for scientific, rather than empirical, control of interim process times during the develop process. Furthermore it is an object of this invention that the improvement of critical dimensions by the adjustment of process times be of sufficient magnitude to justify recharacterization of the process and recalibration of the critical dimensions.
In the develop stage of the photolithographic process, endpoint controllers have been used to determine breakthrough and to adjust the total develop time based upon the time to breakthrough. After breakthrough, the process is commonly allowed to continue for a period called overprocess or overdevelop. During this period, the area of breakthrough is widened and the reaction continues to complete the developing process along the sidewalls of the etched area. It is submitted that the point at which the area of breakthrough is widened and the horizontal development is completed is, in fact the end of the development process and within the develop stage of semiconductor fabrication more accurately called the endpoint and the term will be used accordingly herein.
Heretofore there has been no recognition that endpoint control devices could be used to control portions of the develop process other than the total process time. It is thus another object of this invention to control not only the total develop time, but the interim development stages of the develop process so that the range of variations will be narrowed and will more proximately approach the target or center of the critical dimensions range.
It is axiomatic that the more accurately the process is controlled the more likely it is that the resulting product will be within the critical dimensions and that the yield will be improved. Current develop practice is to select a constant total time and within that total time to empirically select constant times to switch from spray to puddle and from puddle to spray. It is still another object of this invention to teach a develop process wherein variable spray and puddle times are used.
Presently, the total time is the critical parameter used by fabricators. In processing according to our invention, neither the final time nor the interim times are critical. At a point when either a puddle reaches breakthrough or when the chemicals require replenishment, spray begins. Thus interim and total develop times are determined using factors other than time alone, such as the following process changes; percent of time to breakthrough, rate of develop, dissolution rate or chemical depletion rates. A further object of this invention is to find breakthrough and control spray-puddle cycles, independently from the total time of the develop process.
Pertinent references related to endpoint detection of photoresist development include the following: U.S. Pat. No. 4,462,860, End Point Detection, issued Jul. 31, 1984 to Charles R. Szmanda; U.S. Pat. No. 4,501,480, System for Developing a Photo-Resist Material used as a Recording Medium, issued Feb. 26, 1985 to Matsui et al; U.S. Pat. No. 4,647,172, Resist Development Method, issued Mar. 3, 1987 to Batchelder et al; and U.S. Pat. No. 4,136,940, issued Jan. 30, 1979 to Lin.